Most electrolytic cells and fuel cells utilize at least one cathode and one anode in an electrochemical reaction. Typically, a separator keeps a cathode and anode physically separated, and an electrolyte enables electrochemical contact between the two electrodes. In some instances, the separator and the electrolyte are two distinct functional elements. For example, NaCl electrolysis typically employs an asbestos diaphragm or a mercury intermediate electrode as a separator, and a highly alkaline NaCl solution as the electrolyte. However, utilizing a separator and a liquid electrolyte often has many disadvantages. Configurations with an electrolyte and a separator frequently require considerable space. Moreover, liquid electrolytes are often highly corrosive and pose potential leakage problems of the cell with subsequent loss of the electrolyte. Another problem with a liquid electrolyte is that such electrolytes tend to become readily contaminated.
In a polymer electrolyte, the separator and the electrolyte are combined in a single physical component. Generally, a polymer electrolyte carries ion exchange groups, such as sulfonate or phosphonate groups, on a polymeric structure. Depending on the molecular design of the polymer electrolyte, a single ion or an ion pair can migrate through the polymer. The use of polymer electrolytes is advantageous, because electrolysis cells or fuel cells can be configured in various space saving ways. Furthermore, since the polymer electrolyte is not liquid, leakage problems with consequent loss of the electrolyte are typically not encountered.
Many polymer electrolytes, such as poly(ethylene oxide)- and poly(propylene oxide) based compounds, or polysulfone- and polyvinylidene compounds, are relatively inexpensive and can be utilized in various applications. However, some of these electrolytes have relatively low ion conductivity and chemical stability limiting their practicability. Other polymer electrolytes are stable only at relatively low temperatures.
Low temperature stability is especially undesirable, because many electrochemical reactions can be run more efficiently at higher temperatures. For example, the efficiency of water electrolysis benefits from an increase in temperature due to a decrease in the thermodynamic potential and a decrease in electrode polarization. Temperatures in the range of 150.degree. C. to 250.degree. C. are particularly desirable because such temperatures allow for an excellent carbon monoxide tolerance. Moreover, temperatures in the range of 150.degree. C. to 250.degree. C. would enable the direct oxidation of ethanol, other alcohols and hydrocarbons.
To circumvent at least some problems of the low temperature stability, perfluorinated hydrocarbon sulfonate ionomers, such as Nafion.TM. (a perfluorinated hydrocarbon with sulfonic acid groups), have been developed. However, despite their enhanced chemical and thermal stability many difficulties still persist. One problem is that perfluorinated hydrocarbon sulfonate ionomers are relatively expensive. Another problem is that such ionomers tend to decompose at temperatures of about 80.degree. C. and above when they are used over a prolonged period of time.
In recent years, new high temperature polymer electrolytes with improved physicochemical properties have been synthesized. For example, U.S. Pat. No. 5,548,055 and U.S. Pat. No. 5,633,098, both to Narang et al., demonstrate polymer electrolytes based on polysiloxanes and poly (alkylene oxides) with improved plasticity. In another example, U.S. Pat. No. 5,312,895 and U.S. Pat. No. 5,312,876, both to Dang, rigid "rod-type" para-ordered high temperature polymer electrolytes with solubility in water or in aprotic solvents are shown. In a further example, in U.S. Pat. No. 5,741,408 to Helmer-Metzmann, the author shows that the stability of a high temperature polymer electrolyte can be improved by cross-linking polymer electrolyte strands. In a still further example, in U.S. Pat. No. 5,403,675 to Ogata and Rikugata, high temperature polymer electrolytes, such as sulfonated rigid-rod polyphenylenes, are presented that can even operate in the absence of liquid water.
Significant progress in high temperature polymer electrolytes has been achieved with respect to thermal stability and mechanical properties. However, high temperature polymer electrolytes still suffer from a serious disadvantage. Almost all, or all high temperature polymer electrolytes contain aromatic hydrogen atoms that are prone to oxidation, which eventually leads to a decrease in performance and a loss of chemical and structural stability. Therefore, there is still a need to provide improved methods and compositions for electrochemically stable high temperature polymer electrolytes.